Hydraulically operated circuit breaker



Nov. 12, 1963 A. P. STROM ETAL 3,110,783

HYDRAULICALLY OPERATED CIRCUIT BREAKER Original Filed Jan. 16, 1958 2 Sheets-Sheet 1 Fig. I.

WITNESSES: INVENTORS W Albert P. Strom and \4Vilhom H. Fischer.

ATTORNEY Bdliijldd Patented Nov. 12, lhdS ice HYDRAULECALLY @FERATED CERQUH BREAKER This invention relates to circuit breakers and more particularly to hydraulic actuating mechanisms for operating high voltage circuit breakers.

This application is a division of the application of Albert P. Strorn and William H. Fischer entitled Circuit Breaker, Serial No. 709,233, now U.S. Patent No. 311%,- 017, filed January 16, 195-8 and assigned to the assignee of the instant application.

Hydraulic systems utilizing oil as an operating medium have certain inherent characteristics which make them desirable for circuit breaker operating mechanisms. Since liquids are incompressible they can be transmitted through tubes or conduits in all directions without the use of mechanical linkages or other complicated mechanisms. Such hydraulic systems have the advantages that they are smaller and they provide faster controlled operation with less shook. Moreover, because the internal parts are immersed in oil, they are not subject to rust and atmospheric contamination.

An object of the invention is to provide a circuit breaker embodying an improved hydraulic operating mechanism having novel means for controlling the speed of the open-' ing' and closing movements of the movable contact means.

Another object of the invention is to provide a circuit breaker embodying a hydraulic operating mechanism having an improved constant velocity valve for controlling the speed of the moving contact means.

Another object of the invention is to provide a circuit breaker having a hydraulic operating mechanism embodying an improved control valve in which there are no hydrostatic forces acting on the valve.

Another object of the invention is to provide a multipole circuit breaker having a hydraulic operating mechanism for each pole including a cylinder and a piston movable therein for operating the movable contact means to both open and closed positions in which hydraulic pressure is provided by hydraulic accumulators, there being a hydraulic accumulator for each pole mechanism.

Still another object of the invention is to provide a hydraulic mechanism according to the preceding paragraph in which an auxiliary hydraulic accumulator is provided to quickly build up the operating pressure in the individual pole accumulators following an operation of the breaker.

Another object of the invention is to provide a multipole circuit breaker having individual hydraulic operating means for each pole, each operating mechanism having its own hydraulic accumulator, and an auxiliary accumulator common to all of the poles in which the auxiliary accumulator is maintained at a higher pressure than the individual pole accumulators.

Another object of the invention is to provide a multipole circuit breaker having individual hydraulic operating means for each pole in which the operation of all of the pole mechanisms is synchronized by a single control valve.

The invention both as to structure and operation, together with additional objects and advantages thereof,

will be best understood from the following detailed de scription of several embodiments thereof when read in conjunction with the accompanying drawings.

in said drawings:

FIGURE 1 is a diagrammatic view of a hydraulic operating system for a circuit breaker embodying the principles of the invention.

HQ. 2 is a diagrammatic view of a hydraulic operating system for a circuit breaker which system controls a plurality of poles of the breaker from a single pilot valve.

FIG. 3 is a sectional View showing a circuit interrupting device of the puffer type.

Referring to FIG. 1 of the drawings, the circuit breaker is shown diagrammatically at i l and comprises stationary contacts 13 and a movable bridging contact 15. The moving contact is operated to open and closed positions by a hydraulic motor 17 comprising a cylinder 19 and a piston 21 movable therein. A piston rod 23 attached to the piston 21 extends out through one end wall of the cylinder for operating the moving contact 15.

The circuit breaker fill is operated to the open and closed positions by liquid such as oil under pressure admitted to opposite ends of the cylinder l9 at opposite sides of the piston 2 1. In the hydraulic system, energy is stored in a pneumo-hydraulic accumulatorfifi which comprises a cylinder 27 having a movable partition or abutment 2? therein. The chamber 31 beneath the abutment 29 is charged with a suitable gas such, for instance, as dry nitrogen under a predetermined pressure. Oil or other liquid is forced into the accumulator 25 above the abutment 29 by a pump 33 from a reservoir 35 through a conduit37, check valve 39, a conduit dll and a conduit 43. This pressurizes the nitrogen below the abutment 29 thereby providing the operating pressure for the hydraulic system.

The conduit 43 directly connects the accumulator 25 to the cylinder 19 of the hydraulic motor 17 to the left of the piston 21. The accumulator 25 is connected to the righthand end of the cylinder 19 through conduits 45 and 4'7, a closing control valve 49 and conduits 5 2i and 53. A conduit 55 connects the high-pressure conduit 4-7 to a pilot valve 57 which controls a constant velocity opening valve '59.

The valve 49 comprises a cylinder 61 having a valve element as movable therein. Similarly, the pilot valve '57 comprises a cylinder 65 having a valve element 67 movable therein and connected by a rod 69 to the valve element 63 of the valve 49'. The valve elements 63 and 67 are biased toward the right by a spring 71; but, in the closed position of the breaker, they are latched in the position shown by a tripping elect-romagnet 73.

The valve 59 comprises a cylinder or valve housing '75 in which a cup-shaped valve element 77 is slidably movable. The valve element 77 is provided with openings '79 in the bottom 81 thereof and a rod 83 rigidly secured to the center portion of the valve 77 extends in opposite directions out through both end walls of the cylinder 75. A spring 85 disposed externally of the valve cylinder 75 acts through the rod 83 to bias the cup-shaped valve element '77 to the closed position against a valve seat d7 thus shutting off the space above the valve element '77 from an annular space 39 surrounding the side of the valve element; the space 89 being connected bya conduit 91 to the reservoir 35. The space above the valve element '77 in the closed position of the hydraulic system is connected to the accumulator 25 through the conduits 45 and t7, the valve 49, the conduit 51 and a conduit 93.

The lower end of the rod 83 which extends out through the lower end wall of the cylinder 75 is secured to a piston 95 slidable in the bore of a cylinder or piston housing 9'7 which communicates with the bore of the cylinder of the pilot valve 57. A conduit 99 connects the bore of the valve th to the left of the valve element 63 and the bore of the valve 57 to the right of the valve element 67 to the conduit 91 and thence to the low pressure reservoir 35.

it will be noted that due to the piston rod 23 there is a substantial differential in the effective areas on opposite sides of the piston 21 of the hydraulic motor 17, the area on the right or closing side of the piston being substantially greater than that on the left or opening side. Due to this differential in the effective areas of the piston the hydraulic pressure applied to opposite sides of the piston in the closed position maintains the circuit breaker closed.

The breaker is opened by energization of the trip magnet 73 which releases the rod 69 and permits the spring 71 to thrust the valve elements 63 and 67 toward the right causing the valve element 63 to cut off the high pressure oil from the cylinder 27 to the right of the piston 21. At the same time the valve element 67 of the pilot valve 57 opens a communication between the high pressure conduit 55 and the bore of the cylinder 97 permitting fluid under pressure to flow into the cylinder 97 below the piston 95. This forces the piston 95 upwardly and moves the valve element 77 to the open position permitting the high-pressure fluid in the operating cylinder 1.9 at the right of the piston 21. to flow through the valve 59 and conduit 91 to the low-pressure reservoir 35. The highpressure fluid to the left of the piston 21 now moves the piston and the movable contact structure to the open position.

Since the rod 83 of the valve element '77 extends externally of the cylinder 75 in both directions, the hydrostatic forces are equal on both sides of the valve, hence there is no hydrostatic force acting on the valve element in either direction. A constant closing force for the valve element 77 is obtained by means of the spring 85 which is located externally of the valve cylinder. The piston 95 is designed to exert an initial opening force considerably in excess of the closing force of the spring 85, consequently, the valve element 77 is quickly moved to the open position as soon as the pilot valve 57 is operated.

As soon the the valve 77 opens, liquid under pressure will flow through the openings 79. This flow of liquid causes a pressure drop across the opening 79 which is equal to a constant times the velocity squared. it will be noted that there are three forces acting on the valve 77. These forces are the spring 85 and the pressure drop forces which tend to close the valve and the force of the piston 95 tending to open the valve. For the purpose of explanation the spring and piston forces can be considered constant so that the only variable force is that due to the pressure drop across the openings 79. Since the pressure drop is proportional to the velocity of the fluid squared, as the velocity of the fluid increases the valve element 77 will be moved in closing direction thus reducing the velocity of the fluid. Conversely, as the 'velocity of the fluid is reduced the valve element 77 will move in opening direction permitting more fluid to flow. In this manner the valve 77 restricts the flow of fluid to the desired constant velocity. By controlling the velocity of the flow ofrliquid out of the operating cylinder 19, the speed of the operating rod 23 and the moving contact are controlled. Controlled velocity of the moving contacts becomes quite important in a circuit breaker of, for example, the puffer type (FIG. 3) where the forces opposing contact opening vary rapidly and from operation to operation. Without this control the contacts would tend to stall on a severe interruption or permit slamming on a slight interruption.

A closing operation is initiated by energizing a closing solenoid shown diagrammatically at Iltll. Energization of the closing solenoid Till moves the valve elements 63 and 67 toward the left to the positions shown where they are latched up by the tripping magnet 73. In this position the valve element e7 shuts off the high-pressure liquid from the piston 95 and exhausts the pressure from the cylinder 97 below the piston 95 permitting the spring 85 to close the valve 77. At the same time, the valve 4 element 63 permits high-pressure liquid to flow to the operating cylinder 1% to the right of the piston 21. Due to the differential in the effective areas on the opposite sides of the piston 21 the flow of liquid under pressure to the cylinder 17 moves the piston 21 to the left to close the contacts. The parts remain in the positions shown until the tripping magnet 73 receives a tripping impulse.

Hydraulic dashpots, one at each end of the operating cylinder 19, are provided to decelerate the movement of the piston 2i. near the end of both the opening and closing operations. The dashpots comprise plungers ill?) on opposite sides of the piston 21 which, at the ends of the opening and closing strokes, enter restricted openings 1% in the ends of the cylinder. This restricts the flow of fluid from the cylinder on the leading side of the piston 21 and brings the parts smoothly to a stop.

A filter 1697 is provided to filter the oil as it is pumped from the reservoir 35 to the high-pressure system and a conventional relief valve 1% is provided to regulate the pressure in the high-pressure system. The operation of the pump 33 is controlled by a pressure switch 111 responsive to the pressure in the high-pressure system.

The embodiment of the invention shown in PEG. 2 relates to a multipole circuit breaker and has a main openclose valve associated with each pole, the open-close valves for each pole being controlled by a single pilot valve.

Referring to H63. 2 and 3 of the drawings, the circuit breaker is of the three pole type (only one pole being shown) and comprises a pair of operating cylinders 119 each having a piston 121 (only one being shown) movable therein. Piston rods 123 attached to the pistons 121 extend out through one of the end walls of each cylinder 119 for operating a moving contact structure such as is shown in PEG. 3. Each of the contact structures is mounted in a hollow insulator 125 supported on a framework 127, a fragment of which is shown in FIG. 3 and comprises a stationary contact 129 supported on a cap 131 of conducting material rigidly mounted on the outer end of the insulator 125. The stationary contact 129 is engaged by a cup-shaped moving contact l33rigidly secured to the outer end of the piston rod 123 by means of a nut 135 which also secures an insulating cup-shaped arc chamber 137 to the piston rod. The are chamber 137 forms a piston movable in a puller cylinder r39 rigidly supported on the outer end oi' the cylinder 119 and surrounding the piston or contact rod 123. When the mechanism is operated to separate the contacts, the arc chamber 137 is drawn into the puffer cylinder 13% forcing a blast of gas through openings 141 in the arc chamber to extinguish the are drawn between the separating contacts.

in each pole unit, the two contact structures that are operated by the two piston rods 123 (FIG. 2) are connected in series in a manner specifically shown and described in the copending patent application of Albert P. Strom, Serial No. 693,309, now US. Patent 3,075,060, filed October 30, 1957 and assigned to the assignee of the instant application.

The circuit breaker is operated to open and closed positions by liquid under pressure admitted to the operating cylinder Tilt-5 at opposite sides of the piston 12 In the hydraulic system shown in PEG. 2, energy is stored in a main pole pneumo-hydraulic accumulator 143 and an auxiliary accumulator 1145 which arev of the same construction as the accumulator 25 shown in FIG. 1. A pole accumulator 3.43 is provided for each pole of the breaker. Oil is forced into the accumulators M3 and 145 from a low-pressure sump or reservoir 147 by means of a pump through conduit 151 and filter 153. From the pump 3.49 the oil is forced through a check valve 154 and an insulating conduit 155 to the auxiliary accumulator E45 and through conduits 157 and 159 to the pole accumulator M3. This pressurizes the gas in the accumulators to approximately 2000 p.s.i., for example, which provides the h 'draulic operating pressure for the mechanism.

The conduits 157 and 159 directly connect the accumulator 143 to the outer ends of the operating cylinder 11? and the inner ends of the cylinder are connected by means of conduits 161 to an open-close valve indicated generally at 163. The valve 163 comprises a cylinder 165 having spaced inner partition walls 167 and outer partition walls 169 therein. A sleeve valve 171 is slidable in openings in the inner partition walls 167 and the outer partition walls 169 which form valve seats for the opposite ends of the sleeve valve 171. The sleeve valve 171 is provided with a stem 173 the opposite ends of which extend into cylindrical openings 175 in the outer partitions 169. The sleeve valve 171 is biased by a spring 177 into engagement with the lower valve seat in the lower partitions 169. The upper end of the valve cylinder 1% is connected to the accumulator 1 13 by the conduit 159 and the lower end of the valve cylinder is connected by an insulating conduit 179 to the low pressure sump 147.

In order to operate the valve 163 there is provided a pilot valve 181 comprising a cylinder 183 and a valve element 135 movable therein. The valve element 135 is biased by a spring 186 but is normally latched in the position shown by a tripping electromagnet 187, the armature of which engages a latch portion of a valve stem 131*. An insulating conduit 191 and a conduit 193 connect the space 175 below the valve stem 173 of the valve 163 through the pilot valve 181 to the sump 1 37. A conduit 195 connects the high-pressure conduit 155 to the pilot valve 181 but is normally shut oil by the valve element 185.

In the closed position of the parts shown in PEG. 2, highpressure oil is admitted through the conduits 157 and to the outer ends or" the operating cylinders 119 and through the conduit 159, the valve 163 and conduits 161 to the inner ends or" the operating cylinders 119. Due to the differential in the eltective areas on opposite sides of each of the operating pistons 121 the high-pressure oil to the right of each of the pistons holds the associated moving contact 133 (FIG. 3) in engagement with the associated stationary contact 129.

The circuit breaker is automatically opened by energization of the tripping magnet 187 in response to a tripping impulse. Energization of the stripping magnet effects release of the pilot valve element 185, and the spring 186 immediately moves the valve element toward the right shutting oil? the conduit 191 from the conduit and connecting the conduit 191 to the high-pressure conduit 195. This permits highressure oil to flow to the opening 175 below the valve stem 173 which moves the valve 171 upwardly against the upper valve seat on the upper partition 169 and disengages the lower end of the valve 171 from the lower valve seat on the lower partition 169. The high-pressure oil is now cut oil from the operating cylinders 119 on the closing sides of the pistons 121 and the high-pressure oil in these spaces flows through the conduits 161, the valve 1113 and conduit 17% to the sump 147. At the same time, high-pressure oil flows from the accumulator 143 through the conduits 1553 and 157 to the operating cylinders 119 on the opening side of the pistons 121 and moves each of the movable contacts 133 (FIG. 3) to the open position.

In each of the cylinders 1119, together with the arc chamber 137, moves toy as the moving contact 133, Cl is 8 open position, the piston shaped arc chamber moves into the putter cylinder 139 compressing the therein and forcing a blast of gas through the openings 1 11 through the arc chamber to assist in extinguishing the arc.

A closing solenoid 197 is provided toeiiect a closing operation of the circuit breaker. Energization of the closing solenoid 197, which may be effected from any suitable source, moves the valve element of the pilot valve back to the position shown where it is latched by the trip device 187. In this position the cylinder 175 below the valve stem 173 is connected to the low-pressure sump 147 through the pilot valve 181 which permits the spring 177 to move the slide valve 171 downward seat in-g it on the lower valve seat on the partition 169 and disengaging the upper portion thereof from the upper valve seat on the upper partition 1&9. This closes the communications from the cylinders 11h through the conduits 16 1 and 179 and opens the communications between the pole accumulator 143 and the cylinders 119 on the closing side of each of the pistons 121. High-pressure oil now flows through the main valve 163 to the operating cylinders 119 to move the pistons 121 and the movable contacts 133 to the closed positions in the previously described manner.

The circuit breaker includes three pole units each of which comprises the two operating cylinders 119 and pistons 121, the two contact structures (FIG. 3), a pole accumulator 143, the operating valve 163 and a sump 147 all of which are at high electrical potential. For controlling all of the pole units there is provided a pump 1 19 and filter 151 an auxiliary accumulator and a single pilot valve 181 with the trip device 187 and closing solenoid 197 all of which are at ground potential. The single pilot valve 181 is connected to the operating valves 16 3 for the three poles by the insulating conduit 191 and insulating conduits 199. The three sumps 1 17 for the pole units are connected by toe conduit 151 and conduits 201 through the filter 153 to the pump 145-9. The auxiliary accumulator 145 is connected to the highpressure system of each pole by the insulating conduit and insulating conduits 203. A pressure relief valve 2115 is provided to regulate the pressure in the high-pressure system.

In the position of the pilot valve element shown, the operating valve 163 for each of the pole units is connected by the conduits 191, 199 and 193 to the low-pressure sump 14-7. When the trip magnet 187 is energized to release the rod 1&9 the spring 1% moves the valve element 185 to the right and supplies high-pressure oil through the pilot valve 181 and the conduits 191 and 199 to the operating valves 163 for all of the pole units in the previously described manner. All of the pole units are closed by energization of the closing solenoid 197 in the manner previously described. Thus, the operation of all of the pole units is synchronized by the operation of a single, high-speed pilot valve.

Ordinarily, the pressure in the high-pressure system is recovered following an operation of the breaker, during which the pressure in the pole accumulator drops, by operation of the pump, as shown in FIG. 1, which requires a certain amount of time. Provision of the auxiliary accumulator 145 (FIG. 2) makes possible a quicker recovery of the operating pressure in the high-pressure systems of the pole units, than is provided by operation of the pump alone.

By the use or" a pressure regulator shown generally at 207 (PEG. 2) and an additional pressure relief valve 209 it is possible to maintain the pole accumulators 1 13 and the auxiliary accumulator 14-5 at different pressures. For instance, the pole accumulators 1 13 would be maintained at 2000 psi. whereas the auxiliary accumulator 145 would have a higher pressure, for example, 4000 p.s.i. With this higher pressure in the auxiliary accumulator 145, the pressure drop in the pole accumulator 1 13, occasioned by operation of the breaker, is recovered in considerably less time than it would be with both accumulators at the same pressure.

Another advantage of the higher pressure stored in the auxiliary accumulator is that less time is required after operation of the pilot valve to actuate the operating valves 163 thus reducing the opening time of the breaker. This is due to the lengthof the conduits 191 and 1% which requires time to compress the oil therein to provide sufficient pressure to actuate the main operating valves 163. The higher pressure in the auxiliary accumulator causes faster compression and flow of oil through the conduits thus reducing the time required to actuate the operating valves 163.

Certain features of the circuit interrupter are set forth and claimed in United States patent application, Serial No. 693,306, now U. S. Patent No. 3,060,294, filed October 30, 1957, by Winthrop M. Leeds, and assigned to the assignee of the instant application.

Certain features of the circuit interrupter are also set forth and claimed in United States patent application, Serial No. 693,309, now U. S. Patent No. 3,075,060, filed October 30, 1957, by Albert P. Strom, and assigned to the assignee of the instant application.

We claim as our invention:

1. A circuit breaker comprising a pair of relatively movable contacts, hydraulic operating means for said contacts comprising a cylinder and a piston movable within said cylinder to effect opening of said contacts, means for maintaining fluid pressure in said cylinder at one side of said piston to maintain said contacts in a closed position, and constant-low valve means operable to vent fluid pressure from said cylinder at said one side of said piston at a constant velocity to thereby effect opening of said contacts at a constant velocity.

2. A circuit interrupter comprising a pair of relatively movable contacts, hydraulic operating means for said contacts comprising a cylinder and a piston movable Within said cylinder to effect opening of said contacts, means for maintaining fluid pressure in said cylinder at one side of said. piston to maintain said contacts in a closed position, constant-flow valve means comprising a valve housing, a valve member having an orifice therein and movably supported Within said valve housing, a valve stem attached to said valve member and having an end thereof extending through said valve housing, piston means disposed at said end of said valve stem, and means for operating said piston means to operate said constantflow valve means to vent said fluid pressure from said cylinder through said orifice at a constant velocity to thereby effect opening of said contacts at a constant velocity.

3. A circuit breaker comprising a pair of relatively movable contacts, operating means for said contacts comprising a cylinder and a piston movable Within said cylinder to effect opening of said contacts, means for maintaining fluid pressure in said cylinder at one side of said piston to maintain said contacts in a closed position, constant-flow valve means comprising a valve housing, a valve member having a plurality of orifices therein and movably supported Within said valve housing, a valve stem attached to said valve member and having an end thereof extending through said valve housing, piston means disposed at said end of said valve stem, and means for operating said piston means to operate said constantfloW valve means to vent said fluid pressure from said cylinder through said orifices at a constant velocity to thereby effect opening of said contacts at a constant Velocity.

4. A circuit breaker comprising a pair of relatively movable contacts, operating means for said contacts compr' ing a cylinder and a piston movable Within said cylinder to open and close said contacts, means operable to supply fluid under pressure to said cylinder on both sides of said piston to maintain said contacts closed, fluid pressure operated constant-flow valve means operable to vent fluid pressure from one side of said piston to eifect opening of said contacts, and valve means for admitting fluid pressure to operate said constant-flow valve means.

5. A circuit breaker comprising a pair of relatively movable contacts, operating means for said contacts comprising a cylinder and a piston movable Within said cylinder to effect opening and closing of said contacts, means operable to supply fluid under pressure to said cylinder on both sides of said piston to maintain said conta ts closed, fluid pressure operated constant-flow valve means operable to vent fluid pressure from one side of said piston to effect opening of said contacts, said constantflow valve means comprising a valve housing, a valve member movably supported within said valve housing, a valve stem attached to said valve member and having its opposite ends disposed outside of said valve housing means at one of said ends of said valve stem biasing said valve stem and valve member in a closing direction, piston means at the other of said ends of said valve stem, and valve means for admitting fluid pressure against said piston means to operate said constant-flow valve means.

6. A circuit breaker comprising relatively movable contact means, hydraulic operating means for opening and closing said contact means comprising a cylinder and a piston movable therein, said hydraulic operating means being operable to maintain said contact means alternately in the open and closed positions, means operable to supply fluid under pressure to said cylinder on both sides of said piston, said piston having different effective areas on opposite sides thereof, fluid pressure operated constant-now valve means operable to vent the fluid pressure from one side of said piston to effect opening of said contact means, and valve means operable to simultaneously admit fluid pressure to operate said constantiloW valve means and to shut off the flow of fluid pressure to said one side of said piston.

7. A circuit breaker comprising relatively movable contact means, operating means for opening and closing said contact means comprising a cylinder and a piston movable Within said cylinder, means operable to supply fluid under pressure to said cylinder on both sides of said piston, said piston having different effective areas on opposite sides thereof, fluid pressure operated constant-flow valve means for venting the fluid pressure from one side of said piston to effect opening of said contact means, said constant-flow valve means comprising a valve housing, a valve member having an orifice therein and movably supported Within said valve housing, a valve stem attached to said valve member and having its two opposite ends disposed outside of said valve housing, means at one of said ends biasing said valve stem and valve member in a closing direction, piston means at the other of said ends, and valve means operable to shut off the flow of fluid pressure to said one side of said piston and to simultaneously admit fluid pressure to operate said piston means to operate said constant-flow valve means.

8. A circuit breaker comprising relatively movable contact means, operating means for said contact means comprising a cylinder and a piston movable within said cylinder to open and close said contact means, a pneumohydraulic accumulator connected to supply fluid under pressure to said cylinder on both sides of said piston, constant-flow valve means biased closed and operated to an open position to vent fluid pressure from one side of said piston at a constant velocity to thereby effect opening of said contact means at a constant velocity, and valve means operable to admit fluid pressure to operate said constant-flow valve means.

9. A circuit breaker comprisint a pair of relatively movable contacts, hydraulic operating means comprising a cylinder and a piston movable within said cylinder to effect opening of said contacts, means for maintaining fluid pressure in said cylinder at one side of said piston to maintain said contacts in a closed position, constantflow valve means comprising a valve housing, a valve member having an orifice therein and movably supported Within said valve housing, a valve stem attached to said valve member and extending through two opposite ends of said valve housing to prevent hydrostatic forces from acting on said valve member, means outside of said valve housing at one end of said valve housing biasing said valve stem to a position Where said constant-flow valve means is closed, a piston housing disposed outside of said valve housing at the other end of said valve housing, a piston movably supported Within said piston housing and connected to said valve stem, and valve means for admitting fluid pressure to said piston housing at one side of said valve-stem piston to thereby move said valve 7 9 stem against the force of said biasing means to operate said constant-flow valve means to effect opening of said contacts.

10. A circuit breaker comprising a pair of relatively movable contacts, operating raeans for said contacts comprising a cylinder and a piston movable within said cylinder to open and close said contacts, a pneurno-hydraulic accumulator connected to supply lluid under pressure to said cylinder on both sides of said piston to maintain said contacts closed, iiuid pressure operated constantflow valve means for venting fiuid pressure from said cylinder at one side of said piston to effect opening of said contacts, said constant-flow valve means comprising a valve housing, a valve member having a plurality of orifices therein and movably supported Within said valve housing, a valve stem attached to said valve memher and extending through two opposite ends of said valve housing to prevent hydrostatic forces from acting on said valve member, means outside of said valve housing at one end of said valve housing biasing said valve stem to a position Where said constant-flow valve means is in the closed position, a piston housing disposed outside of said valve'housing at the other end of said valve housing, a piston movably supported in said piston housing and connected to said valve stem, and valve means operable to shut oil the flow of fluid pressure to said cylinder at said one side of said piston and simultaneously admit fluid pressure to said piston housing at one side of said valve-sten1 piston toopen said constant-flow valve means to vent fluid pressure from said cylinder at said one side of said piston through said plurality of orifices at a constant velocity to thereby eifect opening of said contacts at a constant velocity.

References Cited in the file of this patent UNITED STATES PATENTS 

4. A CIRCUIT BREAKER COMPRISING A PAIR OF RELATIVELY MOVABLE CONTACTS, OPERATING MEANS FOR SAID CONTACTS COMPRISING A CYLINDER AND A PISTON MOVABLE WITHIN SAID CYLINDER TO OPEN AND CLOSE SAID CONTACTS, MEANS OPERABLE TO SUPPLY FLUID UNDER PRESSURE TO SAID CYLINDER ON BOTH SIDES OF SAID PISTON TO MAINTAIN SAID CONTACTS CLOSED, FLUID PRESSURE OPERATED CONSTANT-FLOW VALVE MEANS OPERABLE TO VENT FLUID PRESSURE FROM ONE SIDE OF SAID PISTON TO EFFECT OPENING OF SAID CONTACTS, AND VALVE MEANS FOR ADMITTING FLUID PRESSURE TO OPERATE SAID CONSTANT-FLOW VALVE MEANS. 